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Author Topic: Bonding and false positive tests on GFCI  (Read 21266 times)

Jeff Bankston

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Re: Bonding and false positive tests on GFCI
« Reply #10 on: September 02, 2014, 01:39:57 am »

I question the wisdom of this preferred method.

First, it would seem to require at least as much effort as providing a good earth ground.

Second, all the careful isolation can be undone with just one careless placement of any metallic tool, stand what have you or even somebody leaning against the trailer .  Maybe in a tightly controlled studio lot (though I am not usually that trusting of coworkers when it comes to my safety)-but very iffy in a venue with public access.

Though LA is probably a drier environment than I am used to so perhaps less susceptible to accidental grounds and harder to get a good ground?
la isnt hard to ground and driving a ground rod is a piece of cake. the exception are if your not near a dirt patch but we never encountered any problems. when a club didnt have the power we needed we used a gen and i clamped the ground wire to the buildings cold water pipe and the inspector approved it.
« Last Edit: September 02, 2014, 01:44:49 am by Jeff Harrell »
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Stephen Swaffer

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Re: Bonding and false positive tests on GFCI
« Reply #11 on: September 02, 2014, 01:41:27 am »

....there would have to be a fault WITHIN the generator itself, that creates an unintentional upstream bond between the generator circuitry and the frame.

A very high percentage of motors I replace have developed a short between the windings and the frame-and most motors have a measurable conductance to ground-especially with a little age.  I would expect the same of a genny-sooner or maybe a lot later there will be a connection like it or not.
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Jonathan Johnson

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Re: Bonding and false positive tests on GFCI
« Reply #12 on: September 02, 2014, 11:55:40 am »

A very high percentage of motors I replace have developed a short between the windings and the frame-and most motors have a measurable conductance to ground-especially with a little age.  I would expect the same of a genny-sooner or maybe a lot later there will be a connection like it or not.

So then the GFI is there not to protect personnel, but to protect personnel in the event of a failure in the generator. I'm sure the advertising execs at Honda will be all over that one.  ;)
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Tim McCulloch

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Re: Bonding and false positive tests on GFCI
« Reply #13 on: September 02, 2014, 12:47:40 pm »

So then the GFI is there... to protect personnel in the event of a failure in the generator.

That's a bad thing?  Not a "feature" the marketing dept will be pushing, but if it provides protection to the user in the event of insulation breakdown or convertor failure I'll be happy the GFI opens.
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Rob Spence

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Re: Bonding and false positive tests on GFCI
« Reply #14 on: September 02, 2014, 01:33:56 pm »

I would imagine that the movie business sets up often enough in locations that would be difficult to drive a ground rod. Concrete streets? Bridges?


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Guy Holt

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Re: Bonding and false positive tests on GFCI
« Reply #15 on: September 02, 2014, 07:08:48 pm »

OK, this doesn't make sense to me. If a generator has a floating neutral (not bonded to ground/frame of genny), then what's the point of a GFI that's factory-installed in the generator?

Almost all portable generators with factory installed master GFCIs have bonded neutrals – you are not missing something. It is the EU Series generators with floating neutrals that do not have GFCIs

The correct way is to follow code….

It is the latest revisions in code that don’t make sense to me. The 2014 edition of the NEC revised the language for section 445.20: Ground-Fault Circuit Interrupter Protection for Receptacles on 15 kW or Smaller Portable Generators as follows:

“All 125-volt, single-phase, 15-and 20-ampere receptacle outlets that are a part of a 15-kW or smaller portable generator either shall have ground-fault circuit-interrupter protection for personnel integral to the generator or receptacle or shall not be available for use when the 125/250-volt locking-type receptacle is in use. If the generator was manufactured or remanufactured prior to January 1, 2015, listed cord sets or devices incorporating listed ground-fault circuit-interrupter protection for personnel identified for portable use shall be permitted. If the generator does not have a 125/250-volt locking-type receptacle, this requirement shall not apply.”

What this means is that, as municipalities adopt the 2014 edition of the NEC, GFCI devices will have to be used on the 125V outlets of generators like the Honda EU6500is, or the new EU7000is, when a spider box or a large HMI light is used on the 240-volt twist-lock receptacle.

While this requirement makes perfect sense when the generator’s 240-volt twist-lock receptacle is being used for home stand by power via a 240V transfer switch, it makes no sense when it is being used to power a large HMI light on set. The reason the revised language of NEC 445.20 makes sense when a generator is used for home stand by power is because neutral and ground are bonded in the service panel of the house. Given this bond, cord set GFCIs plugged into the 125V outlets of the generator will operate reliably because there exists a ground fault circuit for fault current to go to, thereby creating an imbalance in the CT of the GFCI and causing it to trip. However, a large HMI light or spider box plugged into the generator’s 240-volt twist-lock receptacle does not bond neutral and ground, and so cord set GFCIs will not operate reliably because in this situation there is no ground fault circuit for fault current to go to. Regardless of what the NEC says, simply using a GFCI on an ungrounded Floating Neutral generator will not ensure a safe system, and can in fact be misleading because as state previously their test circuits generate a false positive.

Use these links  for more details on the latest code revisions as they pertain to portable generators:

https://soundforums.net/threads/8248-NFPA-decision-on-new-section-NEC-445-20-for-small-(-lt-15kW)-portable-generators

http://www.screenlightandgrip.com/html/emailnewsletter_generators.html#anchorElectrical Hazard Protection

Guy Holt, Gaffer
ScreenLight & Grip
rentals@screenlightandgrip.com
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Stephen Swaffer

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Re: Bonding and false positive tests on GFCI
« Reply #16 on: September 02, 2014, 08:17:10 pm »

Guy, with all due respect you are missing the point of my OP.  The ONLY way someone gets eectrocutes is id current flows through their body, period.  If it returns to the genny via the neutral, the GFCI will never protect them-bonded neutral or not.  IF it finds any other path, the GFCI will trip.  How is this not safer?

I'll to double check, I am on the road with no access to my code book.  IIRC, chapter 250 which always applies unless there is an exception requires "separately derived" systems to have a bonded neutral.  A spider box plugged into the twistlock would qualify as a separately derived system and require a bonded neutral.  You can't factory build a spiderbox that way because, if is plugged into shore power it can not have a bonded neutral. At some point, the end user has to take responsibility for making sure the neutral is bonded when the NEC requires it.  Almost every breaker panel I buy has an unbonded neutral as shipped from the factory-but that doesn't mean I can use it that way.
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Steve Swaffer

Guy Holt

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Re: Bonding and false positive tests on GFCI
« Reply #17 on: September 03, 2014, 08:54:54 am »

Guy, with all due respect you are missing the point of my OP.  The ONLY way someone gets electrocuted is if current flows through their body, period.  If it returns to the genny via the neutral, the GFCI will never protect them-bonded neutral or not.  IF it finds any other path, the GFCI will trip.  How is this not safer?

Stephen, I get your point.  My point is that the false positive generated by GFCI test circuits on generators with floating neutrals, and the revised language of NEC 445.20, mislead the average user into thinking they are using a completely safe system when they are in fact not. Yes, a GFCI on an unbounded generator is safer than no GFCI at all, but a GFCI on a bounded generator that is earth grounded is safer still.


(In an IATSE workshop I create a double fault situation with a fault in the Hot and Neutral conductors)

Take your example that a GFCI will never protect against a Hot to Neutral fault regardless whether the neutral is bonded or not. In an IATSE Local 481 training workshop I offer on Ground Fault Protection, I demonstrate just that. In the demonstration we create a double fault - one in the Hot, and a second in the Neutral.


(Using a variable resistor, we leak current to ground and find that the GFCI does not trip)

We then use a variable resistor to leak current to ground and find that the GFCI does not trip because the fault current returns to the system via the neutral fault before passing through the CT of the GFCI.  As you rightly point out, this is a potentially hazardous situation. If an individual comes into contact with this ground fault circuit created by the two faults, fault current will travel through the individual on its' way back to the generator's windings without the GFCI ever registering a fault.


In a second demonstration illustrated above, we take the same double fault situation, but now bond the neutral and earth ground the generator. The GFCI trips every time. It trips because the ground rod and neutral bond creates a definite Ground Fault Circuit that splits the leakage current - making it impossible for balance to be restored to the system by all of the Fault Current returning to the Neutral through the second Fault before passing back through the GFCI. In other words, by diverting some of the Fault Current around the second fault, the ground rod assures that there will be an imbalance in the current traveling through the GFCI on the return side that will make the GFCI trip. Increasing the effectiveness of GFCIs in double fault situations is a compelling argument I think for, not being lulled into complacency by the false positive of GFCI test circuits, but instead bond and earth ground generators.  For the full content of my IA Ground Fault Protection Workshop, use this link: http://www.screenlightandgrip.com/html/481_GFCI_Workshop.html

A spider box plugged into the twistlock would qualify as a separately derived system and require a bonded neutral.

I don’t see how a spider box plugged into the twist-lock of a generator qualifies as a separately derived system. A separately derived system is a new impedance point where a new voltage is created. A spider box plugged into the twist-lock of a generator is at the same potential as the generator and is therefore a sub panel of the generator system and so should not be bonded.

Guy Holt, Gaffer
ScreenLight & Grip
rentals@screenlightandgrip.com
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Mike Sokol

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Re: Bonding and false positive tests on GFCI
« Reply #18 on: September 03, 2014, 11:25:52 am »

Almost all portable generators with factory installed master GFCIs have bonded neutrals – you are not missing something. It is the EU Series generators with floating neutrals that do not have GFCIs

IIRC there is a UL exception that allow generators under 5KW to NOT have a GFCI if their neutral is floating. But generators with a bonded neutral and 120-volt outlets ARE required to have GFCI protection on the 20-amp outlet(s). However, there are no UL requirements for GFCI protection on 240-volt and higher amperage twist-lock outlets. UL documentation is a little harder to find than the NEC docs since it's specific to each industry, but anyone who builds an electrical device the USA is certainly aware of what it takes to get their products "UL Listed". I'll see if I can find the actual UL docs on this.

The key concept is that a single portable generator powering a single tool or movie light is likely safer if it's both neutral isolated and earth-ground floating. However, once you start distributing power to multiple drop points, such as a stage back-line and a FOH mixing console, you can see how something as simple as a pinched extension cord on stage or a musician plugging in a mis-wired stage amplifier could bias the grounding system to 120-volts above earth potential. So now your mixing guy at the FOH position could be electrocuted if he's standing on the wet grass and touches his gear rack or console.

And while I think that GFCIs are great and should be used for all back-line gear outlets, I don't know of any FOH engineers who would power their mixing console from one. The chance of a stray current leakage taking down an entire show is too much. Same for power amps powered by a twist-lock and a rack-pak. I don't know anyone who put's GFCIs on those circuits since I consider them to be "permanently installed".

I do know that other countries allow 30-mA Industrial GFCIs for power protection since the USA 6 mA threshold has too many trips under normal operation. But I don't think they would pass any inspection here, or am I missing something. 

My take on all this is that for distributed power for pro-stage situations we really want the generator to have a bonded neutral, a properly driven ground rod or bond to building steel, and all metal stage components to be bonded to the same power distro G-N-E connection point. Then there should be GFCI protected outlets for all stage power to offer shock protection to the musicians. If all that is properly implemented then it should be impossible for anyone on stage to suffer a life-threatening shock. Any shock at that point could be from an improperly wired/grounded guitar amp, and the best/easiest way to find that is to point your trusty NCVT (VoltAlert) at every stage amp as it's plugged in a turned on. While Guy may have complete control over his wiring distribution and what lighting gear is plugged into it, in the pro-sound world there's a continuous stream of unknown musicians plugging unknown stage amplifiers in various states of repair into our power distro. So we have to design for the unexpected situations that can put both them and us in danger. 
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Geoff Doane

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Re: Bonding and false positive tests on GFCI
« Reply #19 on: September 03, 2014, 12:12:29 pm »

I would imagine that the movie business sets up often enough in locations that would be difficult to drive a ground rod. Concrete streets? Bridges?

FWIW, I usually see movie generators with a ground wire clamped to the nearest fire hydrant on city locations.  I have no idea if that is 'legal' or not, but I would expect it to work as well as a small diametre ground rod.

GTD
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Re: Bonding and false positive tests on GFCI
« Reply #19 on: September 03, 2014, 12:12:29 pm »


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